Electronic and opto-electronic device manufacturers routinely require commercially grown, large and uniform single semiconductor crystals which, sliced and polished, provide substrates for microelectronic device production. The growth of a semiconductor crystal involves heating polycrystalline raw material to its melting point (in excess of 1,200° C.), bringing the melt into contact with a high quality seed crystal, and allowing the crystallization of the melt. This allows the formation of an essentially cylindrical crystal along a vertical axis with the seed crystal below the polycrystalline raw materials. The equipment necessary to form the semiconductor crystal includes a crystal growth furnace, an ampoule, a crucible, and a crucibles support. The crucible has a lower, narrow portion, called a seed well.
Several problems exist with the conventional crystal growth process and crystal growth equipment. First, controlled incorporation of a dopant such as carbon is often a key factor in the successful growth of semiconductor crystal, such as semi-insulating GaAs materials. Conventional techniques for growing semiconductor crystal, such as GaAs, have proved difficult to achieve controlled incorporation of carbon. These techniques generally involve doping of GaAs by placing a carbon-containing dopant in a GaAs melt and in contact with a molten charge.
Second, crystal growth equipment must be able to withstand extreme temperatures. In addition, the components of the apparatus need to have the rigidity and strength to hold the crucible and growing crystal still in spite of extreme turbulence and convection conditions that often exist within the system at times during crystal growth. Any shift, crack, or movement of a component of the crystal growth apparatus could result in a partial or total loss of the charge. At the same time of providing heat protection and rigidity, the crucible support cannot substantially block heat transfer to the crucible. In fact, precise control of a temperature gradient within the crucible is fundamental to many crystal growth technologies, and the crucible support should not obstruct or impede the heating of the crucible charge.
The temperature gradient control is important in producing a crystal that will yield substrates with uniform electrical properties, as it affects the flatness of the solid-melt interface in the growth process. In order to yield substrates with uniform electrical properties, the solid-melt interface should be as flat as possible. Maintaining the flat interface is difficult because the outer portions of the charge and apparatus tend to cool more readily than the center. For example, at high temperatures, the lower thermal conductivity of solid GaAs (0.7 W/cm.K) makes it more difficult to preserve the planarity of the liquid-solid interface, and slower growth rates are required. Because of the lower thermal conductivity of both liquid and solid GaAs relative to the crucible, the heat conducted by the crucible along the periphery of the ingot becomes more important in determining the interface shape in the conical transition region. As the crystal necks down toward a seed well and the cross section available for axial heat conduction diminishes, a radial temperature gradient arises and the solid-melt interface becomes concave. A method is needed to reduce this radial heat loss in the crucible support to form a flat solid-melt interface just above the seed.
Unless a controlled thermal gradient prevails through the lower, narrow portion of the crucible, nucleation defects, dislocation clusters, lineages, poly-crystal and twin defects tend to form in the transition region. Problems formed in the lower portion of the crucible propagate through the crystal as crystallization progresses up through the charge. On the other hand, if high quality crystal growth successfully extends into the larger diameter portion of the crucible, there is a low tendency of crystal defect formation. Therefore, the ability to control incorporation of a dopant, the quality of temperature control, and the ability to control the vertical temperature gradient and to maintain planar isotherms near the bottom of the apparatus directly affect overall crystal quality and crystal production yield.
In terms of the crucible support and the temperature control gradient, a wide range of solutions is practiced in the industry. Many conventional crucible support solutions involve a solid ceramic structure that might be vented to promote heat transfer by convection or conduction. In general, solid ceramic crucible supports provide effective heat insulation but offer poor temperature control in the lower portions of the crucible. While the strength and stability requirements suggest the need for a solid structure, the heating needs of the furnace in the area of the crucible seed well impose other considerations. A solid ceramic support is inherently unstable because as it expands and contracts, it can crack or shift. Further, the crucible support should not obstruct the flow of heat energy to the raw materials and crystal melt. A solid crucible support would be required to transfer heat from the furnace heating elements to the ampoule and its contents by conduction. Unfortunately, conduction heating is difficult to control at high temperatures, and, in practice, is counterproductive to the creation of a precise, planar temperature gradient as required by many crystal growth technologies.
Similarly, convection heating of the seed well provides imperfect control of the seed well temperature and as a result, also compromises the precision of the temperature gradient in the transition zone of the crucible. Some conventional crucible support techniques, which emphasize heat transfer by convection airflow, form a crucible support from a number of pieces of ceramic rings and spacers. Just as a solid crucible support design detracts from temperature control, a technique based on convection heating by a gas or fluid would similarly fail to meet the specific temperature gradient control needs of most crystal growth technologies, such as Vertical Gradient Freeze (“VGF”).
Conventional techniques based on convection and conduction heat transfer approach fail to provide both precisely controlled heating of the seed well as well as reliable physical stability suitable for volume crystal production.